Low-pressure chemical vapor deposition (LPCVD) reactors are widely used in the manufacture of thin-film devices such as those employed in microelectronic circuitry or in wavelength-selective optical filters. The utilities of such devices frequently depend upon the precision and predictability of their responses to varying inputs. Such responses, in turn, frequently depend critically upon the thicknesses, compositions, and physical properties of the various material thin films of which they are composed. To achieve the required control of thin film chemical and physical properties, the LPCVD processes by which these multilayer devices are manufactured must themselves be carried out under precise control. Further, in order to minimize waste in practical LPCVD manufacturing processes, unpredictable run-to-run variations must be minimized or eliminated. This, in turn, requires that the LPCVD processes be carried out with strict control of the key operating parameters which include, in addition to reactor temperature and pressure, the rates at which the vaporized chemical precursors, oxidants, inert diluents, and other gaseous components are delivered to the operating reactor.
Precise control of the delivery rates of gaseous oxidants (e.g., O.sub.2 or N.sub.2 O) or of inert diluents (e.g., N.sub.2 or Ar) may be achieved quite easily by the use of so-called mass flow controllers which are widely available from a number of commercial sources. Similarly, there are commercially available flow controllers that are designed to deliver various vaporized chemical precursors at controlled rates to the inlet ports of LPCVD reactors. Generally, such chemical precursors are liquids at normal temperatures and pressures. Two distinctly different operations must be carried out so as to achieve a controlled vapor delivery rate--namely, vaporization and flow control. However, these two operations need not be performed in any particular order. Therefore, a gas flow controller may be employed downstream of a liquid vaporizer, or a vaporizer may be employed downstream of a liquid flow controller. Both approaches have been used, and the selection of the approach to be used with a particular precursor is typically based upon a consideration of the chemical and physical properties of the material--most particularly, the so-called vapor pressure curve (equilibrium vapor pressure as a function of temperature), the thermal stability of the material (tendency to thermally decompose as a function of temperature), and sensitivity to the presence of impurities (tendency to react at elevated temperatures with impurities and the effects of such impurities and their reaction products).
A liquid flow controller followed by a downstream vaporizer is most typically employed (and may be required) in cases where relatively high temperatures are required to achieve practical precursor vapor pressures (typically on the order of 5-10 mm Hg), with precursors which have relatively low thermal stabilities, or with precursors which may react detrimentally with impurity molecules (e.g., H.sub.2 O) which might find their way into an evacuated container of the heated precursor. An example of a precursor for which a liquid mass flow controller is, for all these reasons, most desirable is tantalum ethoxide ((Ta(OET).sub.5).sub.2), the reactive precursor most often used in LPCVD processes for the deposition of Ta.sub.2 O.sub.5 thin films.
Perhaps the simplest of liquid flow controllers are those which are basically pumping devices. Included in this category are syringe pumps wherein the movement of a stepper motor is translated into pressure upon the sliding plunger of a liquid-filled syringe. Also included are various other devices such as peristaltic pumps wherein the movement of a rotating shaft translates into the alternate filling and emptying of compartments. The liquid stream exiting from such pumps frequently oscillates or pulsates in velocity, reflecting the alternating mechanical motion by which it is forced through the device. Materials compatibility problems are also frequently encountered, particularly with polymeric seals which tend to adsorb and/or react with the pumped liquid, or which simply wear and eventually leak.
Another class of liquid flow control devices which are used in conjunction with downstream vaporizers are those usually referred to as liquid mass flow controllers (MFC's). By analogy with the more widely used gas MFC's, these devices employ some sort of electronic flow sensing elements along with constant diameter tubing, tuning circuitry, and a feedback loop which operates in conjunction with an electromechanical control valve. Thermal sensors are typically employed, along with heating or cooling devices by which heat is added or removed from the flowing liquid. The measured temperature difference is assumed proportional to both the mass flow and the specific heat of the liquid. Such liquid MFC's are highly sophisticated and relatively complex pieces of equipment with multiple sources of error and possible breakdown points. They must be designed and calibrated for a specific liquid in a specific flow velocity range, and they are only accurate within that relatively narrow range. They typically contain a number of polymeric seals which may begin to leak as a result of absorption or reaction with the flowing liquid, or simply as the result of normal aging and loss of elasticity.
There is, however, another approach to liquid flow control which is, in principle, much simpler than either of the general methods described above. In this simpler approach, a pressure drop is applied across a high-resistance flow control element which might conceptually be nothing more than a tiny orifice or a relatively long length of capillary-bore tubing. The liquid precursor might be contained within a reservoir located upstream of the flow control element. The liquid vaporizer would be positioned downstream of the flow control element between the flow control element and the inlet to the LPCVD reactor.
Unfortunately, there are some basic problems with this approach which, until now, have made it virtually impossible to achieve controlled liquid delivery rates by its use. The first problem derives from the fact that, depending upon the reactor conditions required to carry out a given thin-film deposition process, it may be necessary to deliver the liquid precursor at a very low rate--e.g., a small fraction of a cubic centimeter per minute. Thus, the liquid flow control element must be scaled so as to cause the pressurized liquid to flow through it at very low rates. If an orifice (e.g., a small hole drilled through a metal plate) were employed as the flow control element, the orifice would have to be on the order of 10 microns in diameter. Or, if a length of capillary-bore tubing were used as the flow control device, its length would have to be on the order of 8 feet if its diameter were no more than 10 mils. With dimensions of this type, it is very likely that the flow control device would eventually and unpredictably become partially or completely blocked by small solid particles that somehow find their way into the liquid stream.
A second problem with the attempt to use a pressure drop across a high resistance flow control element to achieve precisely controlled liquid precursor delivery rates derives from the uncontrolled release of dissolved gas on the downstream (low pressure) side of the flow control element. The simplest way to apply a continuously adjustable pressure to the surface of a column of liquid is by the use of a pressurized gas--e.g., by the use of a pressure regulator in conjunction with a source of compressed N.sub.2 or Ar gas. Some of the pressurized gas dissolves within the liquid, the amount of gas dissolved in a particular liquid being determined by the gas solubility at the selected pressure and liquid temperature. Now, since the pressure on the downstream side of the flow control element is much lower than on the upstream side (indeed, compared with the upstream pressure, which may be as high as several atmospheres, the downstream side of the flow control element is practically evacuated), the dissolved gas comes out of solution, forming bubbles of gas distributed randomly within the liquid stream. This process is exactly analogous to the formation of gas bubbles when a bottle of carbonated liquid is first opened. The liquid exiting the flow control element flowing through fairly narrow diameter tubing on its way to the downstream vaporizer then becomes discontinuous as the small gas bubbles combine to form larger ones. Now, the delivery of a constant velocity gas stream from the output of the vaporizer to the inlet of the LPCVD reactor depends upon the delivery of a constant velocity liquid stream from the outlet of the flow controller to the inlet of the vaporizer. The presence of the randomly distributed gas bubbles within the liquid stream, therefore, will cause very large and unpredictable variations in the rate at which the vaporized precursor is delivered to the LPCVD reactor--exactly the opposite of the precisely controlled gas velocity that is required to achieve precise control over the thin film deposition process being carried out within the reactor.
Assuming that this entrained gas bubble problem could be solved, there is yet a third problem with the attempt to use a pressure drop across a high resistance flow control element to achieve precisely controlled liquid precursor delivery to a downstream vaporizer. This problem relates to the difficulty of vaporizing the liquid rapidly and completely such that the vaporized precursor exits the vaporizer at a constant rate that is exactly equivalent to the rate at which the liquid precursor passes through the upstream flow control element. To accomplish this, the slowly moving liquid stream which passes through a fairly narrow diameter tube connecting the downstream side of the flow control element to the upstream side of the vaporizer must drain continuously into the high temperature zone of the vaporizer without forming droplets at the end of the delivery tube and without being held up in relatively low or intermediate temperature portions of the vaporizer--phenomena which typically arise as a result of surface tension and relative wetability effects and which are accentuated by the very low liquid velocities that may be required to achieve the thin-film deposition rates desired in a particular LPCVD reactor process.
Solutions to all of the problems outlined above are contained within the novel apparatus and method for the controlled delivery of vaporized chemical precursor to an LPCVD reactor that are described below.